1. Field of the Invention
The present invention relates to a method for classifying fine particles by using microchannels, and a device for classifying fine particles, which device includes microchannels.
2. Description of the Related Art
Methods for classifying fine particles are categorized into dry methods and wet methods. High accuracy is obtained in dry methods since the difference in specific gravity between a fluid and fine particles is large. In wet methods, although the difference in specific gravity between a liquid and fine particles is small, classification accuracy with respect to fine particles is high because fine particles are easily dispersed in a liquid. A classification device generally includes a rotor in a revolving portion and a stator in a stationary portion of the device, and classifies fine particles by balancing centrifugal force and inertia force. Because of the rotor, however, the classification device has a drawback in that contamination occurs due to abrasion of the rotor, which needs to be removed by cleaning. Further, while dry classifiers which use the Coanda effect and do not have rotors have been commercialized, efficient wet classifiers have not been found. In recent years, various research has been made into performing chemical reactions, unit operations or the like in micrometers, and methods and devices for classifying fine particles efficiently without causing contamination have been studied.
As a method for classifying fine particles, a method is proposed in which the mere introduction of fine particles into a micro-channel enables classification thereof in a horizontal direction in which the fine particles flow. (See, for example, Japanese Patent Application Laid-Open (JP-A) No. 2004-154747, and N201, “Development of Method for Classifying Fine Particles Continuously by Using a Microchannel Laminar Flow System”, Preprints of 69th Annual Meeting by Society of Chemical Engineers, Japan.) In this method, a microchannel (pinched channel) having a tapered portion, where the channel is locally narrower, is used, and a profile of the characteristic flow of fine particles in the microchannel is utilized. It is reported that by this method separation of fine particles with a particle size of 15 μm from fine particles with a particle size of 30 μm is possible. This method, however, has a drawback in that the size of the channel is limited, thereby limiting the particle size. This method also has a drawback in that the amount of particles which can be introduced into the flow is small, thereby making the method inefficient.
Also proposed as a method for classifying fine particles is a separation and classification method, in which a fine particle dispersion is introduced through an inlet into an arc-shaped microchannel having a square cross section, and centrifugal force and lift are used with relation to the flow rate of a fluid and the difference in specific gravity between the fluid and fine particles. (See, for example, N202, “Study on Behavior inside Microseparation Classifier in Accordance with the Euler-Lagrange Method”, Preprints of 69th Annual Meeting by Society of Chemical Engineers, Japan.) Since centrifugal force is used in this method, the larger the difference in specific gravity, the better the classification performance. In the case of fine particles with a small particle size, however, the difference in specific gravity between the fluid and the fine particles is small. Thus, these fine particles are uniformly dispersed in the fluid due to vortex flow in a cross-sectional direction generated by the centrifugal force. For this reason, classification is impossible.
These proposed methods have low classification efficiency and are limited in size and the amount of particles in the flow.